1. Field of the Invention
This invention relates to a resin composition consisting of a block copolymer mixture of vinyl aromatic hydrocarbon and conjugated diene and an impact-resistant styrene resin, and having an excellent transparency and an excellent impact strength.
2. Description of the Prior Art
A variety of production processes and compositions have hitherto been proposed concerning vinyl aromatic hydrocarbon polymer resins typified by styrene resin. Most of the proposals are concerned with improvement in impact-resistance of general purpose polystyrene, because it is brittle and poor in impact-resistance though it is excellent in transparency, surface gloss, beautifulness of appearance, stiffness and tensile strength. There have hitherto been attempted incorporation of a special rubbery copolymer into the general purpose polystyrene, production of an impact-resistant polystyrene resin by graft-polymerization of monomeric styrene on rubbery polymer, production of a styrene-butadiene block copolymer having a high styrene content, etc.
As examples of the incorporation of a special rubbery polymer into general purpose polystyrene, Japanese Patent Publication No. 7126/69 discloses incorporating a linear block copolymer into general purpose polystyrene; and Japanese Patent Application Kokai (Laid-Open) No. 25043/74 and U.S. Pat. No. 3,853,978 disclose a composition of a general purpose polystyrene and a vinyl aromatic hydrocarbon-conjugated diene block copolymer having a special block structure and having a relatively high vinyl aromatic hydrocarbon content. The impact resistance of these compositions are somewhat improved as compared with polystyrene, but cannot be said to be sufficient, and the deterioration in stiffness and transparency is remarkable.
The impact-resistant polystyrene resin in which monomeric styrene is graft-polymerized on a rubbery polymer can be obtained by dissolving a rubbery polymer into monomeric styrene and polymerizing the latter by bulk polymerization, suspension polymerization or by combination of bulk and suspension polymerizations. This type of impact-resistant polystyrene has an excellent resistance to impact. However, it is entirely opaque and poor in surface gloss.
A styrene-butadiene block copolymer having a high styrene content can be obtained by living anionic polymerization. It is formed by polymerizing styrene and butadiene alternately in an inert solvent with an anionic polymerization initiator such as organolithium compound as in Japanese Patent Publications Nos. 3252/72 and 2423/73, etc. This type of block copolymer having a high styrene content is excellent in transparency and superior to general purpose polystyrene in impact-resistance. Though this type of block copolymer fulfils our requirements to some extent, it is still insufficient in impact-resistance, so that the application field thereof is limited.
As mentioned above, among the impact-resistant polystyrenes so far proposed, some are excellent in impact-resistance but are poor in transparency, and the others are excellent in transparency but are poor in improvement of impact resistance. Thus, there has been discovered no impact-resistant polystyrene in which physical properties including impact resistance are well balanced with transparency.
The present inventors previously proposed a styrene resin composition having excellent transparency and excellent impact resistance on the basis of a conception and a knowledge entirely different from those in the past (cf. Japanese Patent Publications Nos. 5,059/78 and 15,958/78 and Australian Pat. No. 488,065). This composition is a styrene resin composition consisting of:
(1) a copolymer consisting of at least one vinyl aromatic hydrocarbon polymer block and at least one conjugated diene polymer block, and having a vinyl aromatic hydrocarbon content of 55-85% by weight, and PA1 (2) a copolymer consisting of at least one vinyl aromatic hydrocarbon polymer block and at least one conjugated diene polymer block, and having a vinyl aromatic hydrocarbon content of 75% by weight or more, or a homopolymer of vinyl aromatic hydrocarbon, PA1 said copolymer (1) being a copolymer consisting of at least one vinyl aromatic hydrocarbon polymer block and at least one conjugated diene polymer block, and having a vinyl aromatic hydrocarbon content of 55-85% by weight and a number average molecular weight of about 10,000 to about 500,000, and PA1 said copolymer (2) being a homopolymer of a vinyl aromatic hydrocarbon or a copolymer consisting of at least one vinyl aromatic hydrocarbon polymer block and at least one conjugated diene polymer block, and having a vinyl aromatic hydrocarbon content of 75% by weight or more, which is at least 5% higher than that in copolymer (1), said copolymer and homopolymer both having a number average molecular weight of about 500 to about 130,000, said copolymer (1) and said copolymer (2) being both obtained by polymerization in an inert organic solvent with an organolithium compound as catalyst;
both copolymers (1) and (2) being obtained in an inert organic solvent with an organolithium compound as a catalyst, characterized in that the weight ratio of copolymer (1)/copolymer (2) is not less than 1.5/1 and that at least 35% of the polymer chain of copolymer (1) is substantially identical in structure with the polymer chain of copolymer (2). Contrary to the prior conception that a mixture of polymers having different compositions is inferior in transparency to the original polymers, this composition is completely transparent and has resistance to impact.
However, even this block copolymer mixture cannot still be said to be fully satisfactory in respect of impact resistance. Although it is comparable or superior in impact resistance to the conventional mixture of general purpose polystyrene and rubbery polymer or to the conventional block copolymers having a high styrene content, a resin having a much more excellent impact resistance has been desired.
For the purpose of improving the impact resistance of a styrene-butadiene block copolymer, attempts have been made to incorporate an impact-resistant polystyrene resin into such a block copolymer (Japanese Patent Publication No. 16,496/77) or to incorporate an impact-resistant polystyrene resin and polystyrene into such a block copolymer (Japanese Patent Application Kokai (Laid-Open) No. 89,550/76). However, the resulting compositions have not necessarily been satisfactory as compositions having both excellent transparency and excellent impact strength.
In view of the above problems, the present inventors have conducted extensive research to find a new styrene resin composition which is excellent in both transparency and impact resistance.